Scanning probe microscopy is a family of imaging techniques in which a tip that interacts locally with a sample is scanned over the surface of the sample to generate a three-dimensional image representing the surface topography and other properties. For example, in atomic force microscopy (AFM) the tip-sample force interactions are measured at each point on the sample. A typical microfabricated AFM probe consists of the cantilever, which is fixed at one end, and the tip attached to the free end of the cantilever. The tip has a very small apex (typically ˜10 nm in diameter) that interacts with the sample. As the tip is moved over the surface of the sample, its force interactions vary in response to the sample topography and are sensed by the cantilever displacement. The cantilever displacement is used to construct AFM images of primarily sample topography that can be acquired either in the contact mode or in the oscillating mode (AC mode). In the contact mode, the tip is brought into contact with the sample and the tip moves up or down as the tip is moved over the surface. The deflection of the cantilever is a direct measure of force and topographical variations. A feedback controller measures the deflection and adjusts the probe-sample separation so as to maintain a constant tip-sample force, i.e. the cantilever is maintained at a fixed deflection. The height of the cantilever's fixed end as a function of the lateral position on the sample is used to construct the final image of the sample's surface.
The applications of the contact mode are limited due to a strong shear force developed whilst the tip is moved over the sample surface while staying in constant contact with the sample surface. These shear forces can damage soft samples. The sample damage can be substantially reduced by operating the microscope in the AC mode. In this mode the cantilever and the tip are driven into an oscillation at a frequency ω1, chosen at or near the resonant frequency of the cantilever. The tip-sample force interactions result in changes of parameters (amplitude, phase or frequency) of the oscillating probe. As the tip is moved laterally over the sample, the controller adjusts the probe-sample separation such that the oscillation amplitude (amplitude modulation technique, AM) or phase/frequency (frequency modulation technique, FM) is kept at a predetermined constant value. Depending on the specific mode, the tip scans over the sample either in the non-contact regime (more often used with FM than with AM) or in the intermittent contact regime (primarily used with AM technique). In both regimes, shearing forces are minimal, and hence, the AC modes can be applied to soft materials. In addition, in the intermittent contact mode, the tip experiences interactions with the sample at much smaller distances than in the non-contact mode, and, hence, resolution can be significantly greater than in the non-contact mode.
The cantilever behavior in AFM is influenced not only by mechanical tip-sample interactions but also by long-range forces such as electrostatic forces between a conducting probe and a sample. Because the same cantilever responds to both forces, their contributions need to be separated. In the non-contact mode this is achieved by operating at two different frequencies. The surface profiling is performed by using the probe oscillating near its resonant frequency while the electric potential is applied to the probe at another frequency ω2. Therefore during scanning, which is performed with the feedback operating at the resonant frequency, the cantilever displacements caused by electrostatic forces are monitored with a lock-in amplifier set at the other frequency. In this way the AFM images simultaneously present sample topography and a map of electrostatic forces. This operation is the essence of electric force microscopy (EFM). In the related approach, known as Kelvin force microscopy (KFM), an additional DC voltage is applied to the probe and the feedback mechanism adjusts the voltage value to nullify the effect of the tip-sample electrostatic force on the probe. In this way, a map of surface potential is generated simultaneously with the topographical image.
In the intermittent contact AC mode, the separation of the mechanical and electrostatic tip-sample force interactions is performed in so-called lift mode. In this two-pass operation the probe behavior is sensed at the same frequency (around the resonant frequency of the probes). In the first pass, the sample topography profile is usually determined in AC mode. In the second pass the tip is moved over the sample being lifted on a small height (5-50 nm) above the sample. In this pass the feedback is off and the tip trajectory follows the just learned surface profile whilst the changes of the probe parameters are caused mostly by long-range electrostatic forces. Both, EFM and KFM can be realized using the lift approach.
At present, EFM and KFM measurements are made with the probe non-contacting the sample surface, and hence, the spatial resolution of mapping of electrostatic responses and surface potential is limited to about 50 nm.